This invention is in the field of transistor amplifier circuits, and relates more particularly to amplifier circuits capable of providing both a wide bandwidth and high gain.
Typical single-stage prior-art differential or single-ended amplifier circuits are capable of providing a wide bandwidth and good high-frequency characteristics, but such simple circuits suffer the drawback of not being capable of providing a high current gain, and may not have a full output swing. Even more complex wideband amplifier circuits, such as those shown in U.S. Pat. Nos. 3,399,357 and 5,045,807, may not supply a sufficiently high current gain or output swing for all circuit applications.
One typical prior-art technique for increasing the current gain of a circuit is to provide a pair of current mirrors, one operating off the "high side" (the power supply bus) and the other operating off the "low side" (ground). A typical current amplifying circuit of this type is shown in FIG. 4.26(b) of ANALYSIS AND DESIGN OF ANALOG INTEGRATED CIRCUITS, Second Edition, Gray & Meyer, John Wiley & Sons, 1977, Page 285. In this self-biased Zener bias reference circuit, a first pair of transistors (Q6, Q7) is connected as the high-side current mirror, with transistors Q2 and Q3 forming the low-side current mirror. However, it is noted that this configuration is used in a DC circuit, and that the necessity for using PNP transistors for Q6 and Q7 in the high-side current mirror would substantially degrade the high-frequency performance of this conventional circuit if it were to be used in a wideband amplifier.
Accordingly, it would be desirable to have a wideband amplifier, in either single-ended or differential form, which would offer high current gain while maintaining a wide bandwidth and good high-frequency characteristics. Additionally, it would be desirable for such a circuit to be simple and compact in design, and economical to manufacture.